Calibration of thermal relays



Jan. 17, 1950 J. J. DIETZ 2,494 863 CALIBRATION 0F THERMAL RELAYS FiledNov. 30, 1944 11a .1? ll j INVENTOR. 39 John J. Dia

Patented Jan. 17, 1950 CALIBRATION F THERMAL RELAYS John J. Diets, NewYork, N. Y.,' sssignor to Thomas A. Edison, Incorporated ,WestOrsnge,

N. 1., a corporation of New Jersey Application November 30, 1944 SerialNo. 565.976

1 "Claims.

This invention relates to improved methods of and apparatus for settingthe timing of thermal relays of the sealed-in type.

This type of relay comprises a thermallyresponsive switch and anassociated heater element for operating the switch, both of which areenclosed in an airtight casing that is substantially evacuated of gas.When the heater element is energized the switch is operated to make orbreak its contacts as the case may be. There occurs however a timeinterval between the instant energization of the heater element is begunand the instant the switch is operated. In different uses of theserelays, diilerent time intervals are required. Typically, the timing mayvary from a fraction of a second to a minute or more.

The setting of the timing of these relays has been diilicult because therelays have to be sealed in before they can be checked. Heretofore, ithas been a common practice to predetermine by experience the spacingbetween the switch contacts and the degree of evacuation necessary tomeet a given timing and to construct the relays accordingly. This methodof setting the timing of the relays is however quite inaccurate and ismoreover inflexible in that it does not lend itself readily to settingthe timing to diilerent prescribed values. According to my invention,however, thermal relays of the character described may be set accuratelyto diflerent timing specifications after they are sealed in, and this isdone reliably and by a very simple method and apparatus.

Among other things, the gaseous pressure in the relay is important indetermining its timing. By my invention 1 provide a novel and effectivemethod and apparatus for externally controlling the pressure after therelays are sealed in.

It is a known principle that the heat conductivity between two spacedmembers in a gaseous ambient varies sharply with the gas pressure atpressures where the mean free length of path of the gas molecules is atleast of the order of the distance of spacing between the two members.By my invention, a novel and practical application is made of thisprinciple for controlling externally the heat conductivity betweendifferent components of sealed-in thermal relays, and it is an object ofmy invention to employ this principle in setting the timing of suchrelays.

It is a further object of my invention to employ an efiective spacingbetween the heater element and associated switch which permits eflectivecontrol of the conductivity therebetween at ranges of evacuation of thecasing which are.

readily attained in practice.

It is another object to provide a novel method of calibrating sealed-inthermal relays, and novel timing of the relay to a value greater thanthat desired, and then release a gas in the casing to lower the timingto that desired value. Preferably, but without limitation thereto, thisgas is retained in the casing in an occluded state, and is released bythermal excitation of the member in which the gas is occluded.

It is an object 0! my invention to provide a novel and efiective meansand method for releasing controlled quantities of such gas.

It is another object to control remotely the release of such gas byelectrical means.

It is another object to retain the gas in a member which serves otheruseful functions in the relay.

Still other objects and features of my invention will more fullyappearfrom the following description and the appended claims.

In the description of my invention reference is had to the accompanyingfigures, of which:

Figure 1 is an axially sectional view of a sealed-in thermal relay takenon the line l--l of Figure 2; and

Figure 2 is a partially axial sectional view of the same relay, taken ina plane at right angles to the view of Figure 1, and showing a circuitsystem connected to the relay for automatically calibrating the samenThe thermal relay shown in the accompanying figures and referred to as Rcomprises a cylindrical casing I, typically of glass, having astem 2 atthe bottom. Sealed into this stem and passing therethrough aresemi-rigid lead wires 3. These lead wires are connected electrically to,and serve in part to support, a thermal switch unit 4 within the casingI.

The switch unit 4 comprises two thermallyresponsive contact members 5and 8 supported in cantilever fashion in a stack I. The stack 1 includesa series of terminals 8 between the two contact members, which areinsulated from each other and from the contact members by interveninginsulating blocks 9 and Ill. At the ends of the stack there are. metalplates II which are insulated from the contact members by insulatingstrips. l2 and clamped together by bolts ll that pass insulatedlythrough the stack. These plates H have wings Ha at the top and bottomwhich bear resiliently against the wall of the casing I to hold theswitch unit 4 in a central position within the casing I.

The contact members and 6 respectively include the contacts 5a and 6awhich, for example, may be normally open as shown. Contact 5a is fixedlysecured to the free end portion of the member 5,but the contact 6a isscrew-threaded to the free end portion of the member 6 so that it may beadjusted to set the gap spacing between the contact members when theyare in their equilibrium positions. The arm portions of these contactmembers comprise identical bimetals of which each is made typically of astrip of Invar applied to a strip of stainless steel. On the bimetal ofthe contact member 5 there is a heater winding I4. This winding is woundtightly onto mica insulating strips l5 which are interposed between itand the opposite sides of the contact member, the mica strips beingsomewhat wider than the contact member to space the winding from theedges thereof. The winding H has circuit connections leading out of thecasing l by way of two of the terminals 8 and two of the lead wires 3.Upon applying a rated voltagetypically 115 voltsto this heater windingfor a period representing the timing of the relay, the contact member 6is deflected sidewise into contact with the semi-stationary contactmember 5 to close the thermal relay.

In order that the thermal relay will be compensated for changing ambienttemperature, the two contact members 5 and 6 are polarized in the samedirection and designed to have the same thermal characteristics. To thisend the contact member 5 comprises a bimetal identical with that of themember 6 and has applied thereto insulating mica strips l6 and asurrounding winding I1, the same as the strips i5 and the winding Hwhich are applied to the member 8. This winding I1 is idle in thefunctioning of the relay, and has been heretofore provided, togetherwith the mica strips [6, solely for the purpose of giving thesemistationary contact member 5 the same resilience as has the contactmember '6 with its associated heater winding H.

There are several heat-flow paths in the abovedescribed relay whichaffect its timing. These are from the heater winding H to (1) the wallof the casing I, (2) the bimetal of the contact member 6, and (3) thebimetal of the contact member 5. Path 1 represents a heat loss and hasthe efi'ect of decreasing the timing-4. e.,-of making the relay workfaster as the heat conductivity of this path is decreased; path 2, tothe contrary, has the effect of increasing the timing-i. e., of makingthe relay work slower-as the heat conductivity is decreased; and path 3is significant principally in that it causes a heating of thesemistationary contact memberi and thereby decreases the timing as theheat conductivity of this path is decreased. It is found that with anincreasing degree of evacuation of the casing I, the timing is notaffected significantly until the gas pressure is reduced within awide-pressure range starting at a maximum value of the order of .2 mm.of mercury. Within this range the timing increases as the gas pressureis reduced, and vice versa. In other words, seemingly, as the heatconductivity within the casing is reduced, the timing is increased,

The heat-flow paths 1 and 3 abovementioned however produce the oppositeeffect. It appears accordingly that in the pressure range just mentionedonly the heat-flow path between the heater winding l4 and its associatedcontact member 6 4 is significant in determining the timing of therelay.

It is believed the heat-conduction path between the heater winding H andassociated contact member 6 significantly controls the timing of therelay within the pressure range mentioned because the mean free lengthof path of the gaseous molecules is, in this pressure range, equal tothe effective length of gaseous spacing of this path, the term gaseousspacing" being herein em ployed to mean the effective distance ofspacing which is free of solid material. The other two paths arerelatively long--being of the order of thirty times as great. In orderfor the mean free length of path of the gaseous molecules to becomeequal to the length of these pathsi. e., to the distance of spacing'fromthe heater winding I4 to the casing I or contact member 5-a very muchhigher degree of vacuum would be required in the casing. It isaccordingly an advantageous feature that one of the significantheat-conduction paths in the relay above described has a widelydifferent length than have the other paths or, in other words, that thepaths which have the same direction of influence on the timing inresponse to a given change in gaseous pressure shall have a widelydifferent length from those paths which 1 ave the opposite direction ofinfluence on the timing. Also, it is important that one of these pathsshall be relatively short so that the timing will be controlled withouthaving to obtain a high degree of vacuum.

The regions of gaseous spacing between the heater winding [4 and contactmember 6 are localized between the heater winding and the narrower sidesof the contact members and between the mica strips l5 and the broadersides of the contact members... This latter spacing regionwhich isbelieved to be the main one in importance-arises because the heaterwinding is in direct contact with the mica strips along the widththereof but the central portions of the mica stripsare bowed outwardlyaway from the contact members by the tensioning of the heater winding onthe portions of the mica strips which overhang the edges of the contactmembers. The spacing accordingly varies at different points, and theeffective spacing is a mean value of the limits between which thespacing varies. It is considered beneficial that the spacing is not welldefined-4. e., not uniform-because this has the effect of giving a moregradual control over the timing with change in the gas pressure.

As a typical example, the bimetal of the contact member 6 may beapproximately .930" thick and .250" wide, the mica strips [5 may beapproximately .0O8" thick and .375" wide, and the winding I! may have atensioning o! the order of .75 lbs. For these values, I find theconductivity between the heater winding l4 and the contact member 6 isinfluenced significantly by'the gas pressure at pressures of the orderof .2 mm. of mercury and less. Pressures of such low order are herein.referred to generally as representing a substantially evacuatedcondition.

Since the heat conductivity between the heater winding i4 and thecontact member 6 varies sharply with variation in gaseous pressurewithin the range abovementioned, it follows that small variations in thequantity of gas within the casing I are eflective to produce widechanges in the timing of the relay. In carrying out my invention, Iinitially over-exhaust the casing so that the timing of'the relay willticularly as they affect the spacing between. the

heater winding l4 and the contact member 6. cause the initial timing tovary within a range of values. Purely by way of example, for a given setof relays this range may be from 45 to 60 seconds, and the margin maytherefore be seconds.

My invention employs a means within the casing i which will produce'orrelease an amount of free gas-typically hydrogen-according to the degreeto which, and the length of time, that means is heated. Preferably, thegas is made available by being stored within the casing in anoccludedstate. For this purpose, I preferably use the normally idle winding H asthe gasretaining means. In order that this winding may be heated byexternal means to drive off portionsof the gas therefrom, leadconnections. are brought out therefrom through the casing l.

p of circuit 20, and the tap 21a is connected through relays 23 and 29to the high-potential side 2 la of circuit 2 l. Thus the voltagessupplied to the respective circuits 2!! and 2! may be adjustedindependently of each other to desired values as indicated by respectivevoltmeters Vi and V2; also, it will be seen that the relay 2% serves asan on-ofi control not only for the circults 20 and M but for the entiresystem as will hereinafter appear.

The circuit 20 has lead connections 39 and to those lead wires 3 of therelay R, which con nect to the heater winding is and the circuit it haslead connections 32 and 33 to those lead wires 3 which connect to thegas-retaining wind ing i1, one of these lead wires being to the corntact member 5 as above explained. Also, in order that the energizationof the winding it will beterminated when the relay R is closed, thehigh-potential side 2| a of the circuit 2! has a lead connection 34 tothe contact member ii, the action of .this connection being to provide ashort circuit around the winding I! when the relay R closes. However, inorder that there However, only one extra lead connection is: re- 25,will not be 8 direct Short Placed 011 e quiredwhich is the leadconnection by way of when the relay R 610588, there s s y one of theterminals 8 and lead wires 3 aforementionedsince the winding i1 isconnected at I 8 to the contact member 5 and the lead connection to thiscontact member serves also as a lead connection to the winding.

The method of charging the winding IT with occluded gas is as follows:The casing i is first connected to a vacuum line for a period ofapproximately 10 minutes during which time voltage ls applied to thewinding I! to cause it to glow a bright red. Hydrogen gas toapproximately 760 mm. pressure is next introduced into the casing, andthe voltage to thewinding i1 is V adjusted so that it will again glow abright red. After a few minutes of this excitation, the voltage isremoved from the winding and the winding is left to cool in the hydrogengas. The casing is then again connected to a vacuum line to reduce thepressure to a value of the order of .001 mm. of mercury. At the sametime voltage is applied to the heater winding 14 to cause it to glow abright red so as to drive out the occluded gas therefrom. Thisevacuating of the casing I and energizing of the heater winding I4arez'continued for approximately 30 minutes, after which the voltage isremoved and the casing l is sealed.

The initial timing of the relay R may be assumed to be 45 seconds andthe final desired timing to be 30 seconds. The reduction in the timingto this desired value is carried out aut'o matically by the timingsystem shown inFigure 2. This timing system includes two ener'- gizingcircuits 20 and. 2! having a common return lead 22 which may beconsidered as the low or ground side of the system. The high potentialside of the circuit 2|] is accordingly the lead 20:: and that of circuit2| is the lead 2m. These circuits derive their voltages from a commonsource (not shown) by way of a sup ly line23 and a manual on-off switch24 for the system. One side of the supply line is connected through arelay 25 to the ground side 22 of the" circuits 20 and 2! and the otherside of the supply line is connected through respective voltage-adjust-'ing devices 25 and 21 to the ground side 22, the devices 26 and 21 beingauto transformers having variable taps 26a and 21a, these devices beingknown commercially as Variacs. The tap 26a is connected to thehigh-potential side 20::

eluded a resistor 35 in the lead connection 32 from the low-potentialside 22 to the contact member 5. The resistance 35 is small in relationto the resistance of ,the winding ll, a typical value being 100 ohms fora winding of 2000 ohms.

The relay 25 is normally closed, and this being the only relay in thecircuit 20, this circuit is energized the instant current is applied tothe system. The relay 2!! is also normally closed, but rela 231snormally openit being understood that the term "normally is here appliedto mean the condition of the relays when they are unenergized and atambient temperature. Thus ethc circuit 2i is initially unenergized whencurrent is applied to the system. Each of the relays 25, 28 and 28 havestationary contact arms and tomperature-responsive contact armsdesignated respectively by the reference numerals of the relay with thesufllx letters a and b. Associated with the movable arms of the relaysare heater windings designated by the reference numerals of the relayswith the suffix letter 0. The heater of relay 28 is connected acrosscircuit 20 by leads 3'5 and 31 and is therefore energized the instantcurrent is initially supplied to the system. This relay has a,make-contact timing period equal to that desired for the relay B. beincalibrated, this being 30 seconds in the present instance as abovementioned. Thus, it will be understood that the timing system operatesto energize the heater winding It the instant current is supplied to thesystem and that when the winding l 4 is energized for a. period equal tothe timing period desired for the relay R, circuit 2| is closed by relay28 to supply energizing current to the gas-retaining wind- 7 ing i1 andto start the release of gas therefrom.

When the relay 28 closes, energ'zing current is supplied to the heaterwindings of the relays 25 and 29 from the circuit 2|. The energizingcircuit for the relay 25 is from the movable contact 28b of relay 28through a lead 38, heater winding 250, a connection 25d of this windingto the contact 251), and the contacts of the relay 25 to the low side ofthe supply line 23, and the energiz-' 7 to relay 2 and through thisrelay to the low side of the supply line 28. Thus. the instant relay IIcloses to start the energization of the gas-retaining winding l1, relaysI! and II are energized and begin moving to open positions. Relay 29controls the length of time the gas-retaining winding is energized, andmay suitably have a-timing of 8 seconds. It is desired that relay IIshall have a timing Just longer than that of relay 2!, a suitable valuebeing 10 seconds. Accordingly, it will be understood the gas-retainingwinding I1 is energized for a period of 8 seconds, beginning when theheater winding H has been energized for the timing period desired forthe relay R; after the lapse of an additional 2 seconds following thetermination of energization of the winding l1,-

during which the heater winding It continues to be energized, relay 2!opens to shut off the current supply to the entire system. Relay 2! hasa long contact-make timing period to allow the relay R to cool andreturn substantially to ambient temperature, a suitable timing for thisp pose being 3 minutes. The contact-make period of the relay 29 and thecontact-break period of relay 28 are each relatively short andaccordingly these relays are respectively in closed and open conditionsat the time the relay 2! recloses.

The closing of relay 25 represents the completion of a cycle ofoperations of the automatic timing system, or in other words anenergizing cycle for the relay R being calibrated, it being noted thatthis cycle comprises energizing the heater winding I for the desiredtiming period (30 seconds) of the relay R, a continuing energization ofthis heater winding together with a concurrent energization of thegas-retainingwinding I! for an additional period (8 seconds), andthereafter a deenergization of the timing system and the relay R for aperiod (3 minutes) to allow the relay R and the relays of the timingsystem to return to their initial positions. Thereupon, with thereclosing of relay 25, a second energizing cycle is started, and so on.

Each 8-second period of energization-of the gasretaining winding II willcause a quantity of free gas to be released in the casing l and willcause the timing of the relay R to be reduced by a deflnite period,typically 3 seconds. Thus, assuming the relay R has an initial timing of45 seconds, it will have a timing of 42 seconds at the completion of thefirst energizing cycle.- At the completion of a second energizing cycle,the timing of the relay will be reduced approximately another 3 secondsto 39 seconds.

the contact member I is negligible. We may assume the deflection of thecontact member I to be sufficiently large during the third cycle hereconsidered that the relay R will not close before the relay 28 opens. Asa result, the timing of the relay R is reduced during this thirdenergizing cycle by another 3 seconds, bringing the timing of the relayto approximately 36 seconds.

During the fourth and succeeding energizing 10 cycles, the period ofenergization of the gas-retaining winding ll becomes shorter andshorter, the amount of gas released during each cycle is less and less,and accordingly the timing of the relay R is reduced by smaller steps,the timing 5 approaching asymptotically the predetermined 30-secondperiod as a limit. (In view of the inherent thermal delay in the windingll reaching a temperature whereat it will release gas when it-isenergized, the timing of the relay 28 is desirably set to a valuesomewhat less than that desired for the relays being calibrated.) In nocase, however, can this system operate to reduce the timing of the relayR below this limit. Accordingly, it will be understood that a series ofrelays to be calibrated to the same timing may be connected in parallelto the circuits 20 and 2| and that even though they have differentinitial timing periods varying over a wide range, all will be brought tothe same timing by the 30 present calibrating system.

While I have herein particularly shown and described my invention interms of a preferred embodiment thereof, it will be understood that thisembodiment is illustrative and not limitative of my invention as thesame is subject to changes and modifications without departure from thescope of my invention, which I endeavor to express according to thefollowing claims.

. I claim:

1. The method of reducing the timing of a sealed-in thermal relay to apredetermined value, said relay including a substantially evacuatedcasing and a temperature-responsive contact member, which comprisesstoring a gas in said casing out of contact with said members, andreleasing a quantity of said gas according to the differential betweenthe initial timing of the re-- lay and said predetermined value.

2. The method of reducing the timing of a sealed-in thermal relay, saidrelay including a substantially evacuated casing and atemperature-responsive contact member, which comprises occluding a gaswithin said casing, and driving of! portions of said occluded gas intothe Durin the third energizing cycle, the timing free Spa within d 0 8-of the relay R is reduced to less than 38 seconds and the relay R mayclose before the B-second timing period of the relay 2! elapses to stopenergization of the gas-retaining .winding ll. If this should happen,the energizing period of the gasretaining winding is cut short, sincethe winding I1 is effectively shorted through resistor 35 when relay Rcloses as hereinbefore described. However, the temporary energization ofthe gasre- 3. The method of reducing the timing of a sealed-in thermalrelay, said relay including a substantially evacuated casing and atemperature-responsive contact member, which comto prises occluding gaswithin said casing, and thermally releasing portions of said gas intothe free space within said casing.

4. The method of reducing the timing of a sealed-in thermal relay, saidrelay including a taining winding I1 produces some deflection ofsubstantially evacuated casing and a temperathe contact member 5 awayfrom the other contact member 6, and tends therefore to delay theclosing of the relay R. The deflection of the contact member 5 has theveffect therefore of hastenture-responsive contact member, whichcomprises occluding a thermally releasable gas within said casing, andelectrically heating said gas to release portions thereof into the freespace within ing the action of the timing system, but this holds saidcasing. a

5. The method-of setting the timing of a sealed-in thermal relay havinga casing, a tem-.

perature-responsive contact member and an associated heating memberhaving a. predetermined log are gradually .shorterand the deflection ofeilective distance of spacing from said contact member, which comprisesevacuating said casing to a pressure whereat the mean free length ofpath of the gaseous molecules in said casing is substantially greaterthan said etlective distance of spacing between said contact and heatingmembers, and producing a free gas within said casing to reduce said meanfree length of path to the order of magnitude of said distance ofspacing.

d. The method of setting the timing of a sealed-in thermal relay havinga casing, a temperature-responsive contact member and an associatedheating member having a predetermined eiiective distance of spacing fromsaid contact member, which comprises storing occluded gas in a memberwithin said casing, evacuating said casing to a pressure whereat themean free length of path of the gaseous molecules in said casing issubstantially greater than said efiective distance of spacing, andheating said storing membe: to release a portion of said occluded gaswhereby said mean free length of path is reduced to the order ofmagnitude of said distance of spacing. I1

7. In a thermal relay including a substantially evacuated casing,atemperature-responsive contact member and an electrically-energizableheating member associated with said contact member for operating saidrelay: the combination of an electrically-energizable heating memberwithin said casing, saidv latter heating member having occluded gasstored therewithin; and circuit element leading out of said casing fromsaid latter heating member whereby the heating member is electricallyenergizable to cause portions of said gas to be released therefrom.

8. In a thermal relay including a substantially evacuated casing, a pairof similar contact members within said casing, said contact membersincluding bimetals polarized in the same direction, and a heatingwinding on one of said members for operating said relay: the combinationof a second heating winding provided on the other of said contactmembers for imparting thereto a resilience equal substantially to thatof said one contact member, said second heating winding having occludedgas stored therewithin; and circuit means leading out of said casing forelectrically energizing said second heating winding to release portionsof said occluded gas therefrom.

9. The method of reducing the timing of a thermal relay to apredetermined value, said relay including a substantially evacuatedcasing, a thermally-responsive switch, and a gas-retaining meanselectrically energizable to cause a free gas to be released within saidcasing, which comprises subjecting said relay to successive energizingcycles of which each cycle is for a time period greater than saidpredetermined value, energizing said gas-retaining means for a timeinterval within said period beginning when said relay has been energizedfor a time duration equal to said predetermined value, and thereafterdeenergizing said relay and gas-retaining means.

10. The method of reducing the timing of a thermal relay to apredetermined value, said relay including a substantially evacuatedcasing, a thermally-responsive switch, and a gas-retaining meanselectrically energizable to cause a free gas to be released within saidcasing, which comprises subjecting said relay to successive energizingcycles of which each cycle is for a time period greater than saidpredetermined value, energizing said gas-retaining means when said relayis energized for a time duration equal to said predetermined value,stopping energize,- tion of said gas-retaining means during said periodat a fixed time interval following the expiration of said predeterminedvalue or upon operation oi said thermal switch in response toenergization of said relay depending upon which occurs the sooner, andsubsequently stopping energization of said relay to allow the same tocool.

11. The method of reducing the timing of a thermal relay to apredetermined value, said relay including a, substantially evacuatedcasing, a thermally-responsive switch, and a gas-retaining meanselectrically energizable to cause a free gas to. be released within saidcasing, which comprises subjecting said relay to successive energizingcycles or which each cycle includes simul-- taneously energizing saidgas-retaining means upon said relay being energized for a time durationequal to said predetermined value, stopping energization of said gasretained means upon operation of said thermal switch, and subsequentlystopping energization of said relay until the same and saidgas-retaining means are restored substantially to ambient temperature.

- ,12. The method of reducing the timing of a thermal relay to apredetermined value, said relay including a substantially evacuatedcasing, athermally-responsive switch, and a gas-retaining meanselectrically energizable to cause a free gas to be released within saidcasing, which comprises subjecting said relay to successive cnergizingcycles of which each cycle includes simultaneously energizing saidgas-retaining means for a fixed time interval beginning when said relayis energized for a time duration equal to said predetermined value, andsubsequently stopping energization of said relay to allow the relay tocool and return substantially to ambient temperature.

13. A method of reducing the timing of a thermal relay to apredetermined value, said relay including a substantially evacuatedcasing, a switch including a pair of temperature-responsive contactmembers, and a gas-retaining means electrically energizable to releasefree gas within said casing which comprises subjecting said re-= lay tosuccessive energizing cycles of which each cycle comprises applying agiven voltage to said relay, simultaneously applying a predeterminedenergizing voltage to said gas-retaining means, the energization of saidgas-retaining means being started upon said relay being energized for atime duration equal to said predetermined value, stopping energizationof said gas-retaining means upon operation of said switch, andthereafter deenergizing said relay.

14. A thermal relay comprising a movable temperature-responsive contactmember, a relatively stationary cooperating contact member, anelectrically-energizable heating member thermally associated with saidmovable contact member for operating said relay, an enclosing casing forsaid contact members and heating member, and a gas medium in saidcasing, said relay being characterized as having heat-flow paths ofdifferent lengths from said heating member which respectively influencethe timing of the relay in different directions, the pressure of saidgas medium in said casing being set to a value whereat the mean freelength of path of the gaseous molecules is of the order of the effectivelength of the gaseous spacing in that of said heat-flow paths havingunidirectional timing influence on said relay.

15. In a system for automatically timing a assures ll sealed-in thermalrelay, said relay including a substantially-evacuated casing, athermally-responsive switch, a heater for operating said switch andgas-retaining means electrically energizable to release free gas withinsaid casing: the combination of a first circuit for energizing saidheater; a second circuit for energizing said gasretaining means; meansfor connecting said circuits to a source of electric current; a firstcontrol switch in said second circuit having a normally-open position;means, connected to said first circuit and operated by currenttherefrom, to close said first control switch at the expiration of apreset interval; a second control switch serially included in saidsecond'circuit and having a normally-closed position; means, renderedoperative by said first switch as the same is closed, to

open said second switch at the expiration of a preset interval followingthe' closing of said first switch; and means, timed with saidlast-stated means, to deenergize said system following the opening ofsaid second switch.

18. In a system for automatically timing a sealed-in thermal relay, saidrelay including a substantially-evacuated casing. a thermally-responsiveswitch, a heater for operating said switch and gas-retaining meanselectrically energizable to release free gas within said casing: thecofiiks bination of a first circuit for energizing said heater; a secondcircuit for energizing said gasretaining means, a first thermal-typecontrol relay having normally-open contacts in said second circuit andhaving a heater winding energized by said first circuit, said relaybeing adapted to close to supply energizing current to saidgas-retaining means when the heater winding thereof has been energizedfor a preset interval; means for connecting said circuits to a sourcecircuit by said second nd circuit when of electric current; a secondthermal-type control relay having normally-closed switch contacts insaid second circuit and having a heater winding energized by way of saidconnecting means and said contacts of said first control relay when saidconnecting means and first control relay are closed. said second controlrelay being adapted to open said second circuit to cut oil the currentsupply to said gas-retaining means at the expiration of a presetinterval following the closing of said first control relay; and a mastercontrol thermal-type relay for said system having normally-closed switchcontacts serially included in said first and second circuits and havinga heater winding energized by way of said connecting means and saidcontacts of said first control relay when said connecting means andfirst control relay are closed. said master relay having a contact-breakperiod at least greater than that of said second control relay wherebyto deenergize said system following the opening of said second controlrelay.

17. The combination set forth in claim 16 comprising a circuit seriallyincluding said thermany-responsive switch for disabling said secsaidthermally-responsive switch is closed.

JOHN J. DIETZ.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,245,168 Suits June 10, 19412,310,747 Payne Feb. 9, 1943 2,369,619 Stibitz Feb. 13, 1945 Certificateof Correction Patent No. 2,494,863 January 17 1950 JOHN J. DIETZ It ishereby certified that error appears in the printed specification of theabove numbered patent requiring correction as r'ollows:

Column 9, line 33, for element read elements; column 10', line 21, forgas-retained read gas-retaining;

and that the said Letters Patent should be read as corrected above, sothat the same may conform to the record of the case in the PatentOflice.

Signed and sealed this 23rd day of October, A. D 1951.

[SEAL] THOMAS F. MURPHY,

Assistant Uom'mz'ssioner of Patents.

Certificate of Correction January 17, 1950 Patent No. 2,494,863

JOHN J. DIETZ It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction asfollows:

Column 9, line 33, for element read elements; column 10", line 21, forgas-retained read gas-retaining; and that the said Letters Patent shouldbe read as corrected above, so that the same may conform to the recordof the case in the Patent Otflice.

Signed and sealed this 23rd day of October, A. D. 1951.

[SEAL] THOMAS F. MURPHY,

Assistant Commissioner of Patents.

